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"How much did Santa's sleigh cost? Zero, it was on the house."

This one-liner is greeted with groans that resonate through a storage facility in the capital.

We're at a humor-evaluation session with a firm that makes products for social events. Its repertoire features festive crackers.

The company's founder grins, nearly apologetically at the joke. But the joke has been selected and will feature in future crackers.

"The success is gauged by the gag by the number of groans and the intensity of the groans at the table," the founder explains.

The secret to a good holiday cracker pun is not the identical as a good gag per se. It is entirely about the context - in this instance, the communal laughter of the holiday meal with grandparents, kids and potentially friends.

"The goal is for the joke to be a thing that unites the child together with the 80-year-old," she adds.

The Science Behind Communal Amusement

Coming together to enjoy shared amusement is not only nothing new, scientists say, it is probably to be older than humanity.

"So when you are chuckling with others at the holiday table you are engaging in what's almost certainly a really primordial mammal play vocalisation," explains a neuroscience expert.

Shared laughter, she explains, helps make and maintain social bonds between people.

Scientists have found that a absence of such social exchanges can seriously harm mental and physical well-being.

"Those you converse with, and laugh with, it leads to enhanced levels of 'happy chemical' uptake," the professor adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with loved ones over a particularly awful Christmas cracker joke.

"It's not simply laughing at a silly pun with a holiday cracker," she says. "You are in fact doing a lot of the truly vital task of building, preserving the connections you have with the people you care about."

Which Happens In the Brain?

But what is truly happening inside the brain when we hear a gag?

A tremendous amount happens in response to comedy, it transpires.

Employing brain scanning technology, a type of brain scanner which indicates which parts of the brain are working harder, researchers have been able to chart the areas that receive more blood.

Testing involves scanning the brains of volunteer subjects and then subjecting them to a database of funny words, accompanied by either a neutral sound, or pre-recorded laughter.

"During the study we observed a very fascinating pattern of activation," says the professor.

A gag stimulates not just the areas of the brain responsible for hearing and interpreting language, but also neural areas involved in both preparation and starting motion and those linked to vision and recall.

Put all of this together, and people listening to a pun have a sophisticated set of brain responses that support the laughter we experience.

The Infectious Nature of Laughter

Researchers discovered that when a funny word is paired with chuckles there is a stronger reaction in the brain than the identical phrase when followed by a neutral sound.

"This activation occurred in areas of the mind that you would employ to move your expression into a smile or a chuckle," the professor explains.

It indicates we are not just responding to funny words, they are responding to the amusement that accompanies them.

Amusement, says the expert, can be infectious.

So what does this mean for the chuckles heard around a Christmas gathering?

"You laugh more when you know people," she notes, "and laughter increases further when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she explains, the feel-good effect is more probable to be caused not by the gag in itself, but from the response to it.

"It's the laughter. The gag is the dreadful holiday cracker pun, and it's just a pretext to chuckle together."

The Search for the Ideal Cracker Joke

Is it possible to find the ultimate gag?

Likely not, but that has not prevented researchers from trying to.

In 2001, a professor set up a research project for the world's funniest gag.

Over tens of thousands of jokes later, with ratings provided by 350,000 people around the world, he has a better understanding than many as to what works and what fails.

The ideal Christmas cracker joke needs to be brief, he explains.

"But they also be poor gags, jokes that cause us to groan," he continues.

The more "awful" the gag, he states the more effective.

"This is because if no-one finds it funny – it's the gag's fault, not yours.

"The fascinating part about the Christmas cracker puns is that not one person considers them humorous.

"It creates a shared experience at the gathering and I think it's wonderful."